The present invention relates to tree assemblies commonly referred to as trees, and more specifically to a tree suitable for subsea use having a spool with a lateral production port.
Subsea wellhead assemblies and production trees are used in the oil and gas industry for recovering hydrocarbons, with the assembly conventionally supporting a production tubing within a well below a blowout preventer which defines therein a BOP bore. The blowout preventer is conventionally connected to one or more fluid lines extending from the BOP to the surface. A workover string extending from the surface to the BOP is used for fluid communication with the BOP bore. Subsea production assemblies may generally be classified as conventional or vertical production trees, single bore trees which also include vertical production, and horizontal trees. A single bore tree is disclosed in U.S. Pat. No. 5,143,158.
Horizontal trees differ from a standard Christmas tree for an oil or gas well with respect to the spool body of a horizontal tree which is connected above a wellhead housing typically mounted at the upper end of a casing string, and a tubing hanger landed within the housing to suspend a production tubing string within the casing. The spool has production and annulus valves to control fluid flow from their respective areas, and landing the tubing hanger in the spool body allows the hanger and tubing to be removed without removal of the spool body. This provides a significant advantage over more conventional trees where there is a risk of having to pull the tubing, since the valves located on the tree provide access to the production and annulus bores once the tree is installed. Additional valves are required during the installation phase to allow access to the tubing/casing annulus after the tubing hanger is landed and sealed to the tree body.
Commercially available horizontal trees include at least three ports from the spool body: a production port for production fluid; an annulus port for communication between the production tubing and the wellhead housing, and a workover port in communication with the BOP bore, and thus with the workover string extending to the surface.
U.S. Pat. Nos. 5,544,707 and 6,039,119 disclose horizontal tree assemblies. Other patents of interest include U.S. Pat. Nos. 4,853,611; 5,465,794; 5,555,935; 5,582,438; 5,706,893; 5,730,473; 5,749,608; 5,865,250; 5,868,204; 6,050,339; 6,062,314; 6,119,773; 6,158,716; 6,244,348 and 6,302,212. Another version of a horizontal spool tree assembly is depicted on page 44 of the Dril-Quip 2000 General Catalog. U.S. Pat. No. 6,378,613 discloses a Christmas tree and tubing hanger system. Other relevant publications directed to oilfield technology, including gate valves, include WO 01/73258; WO 01/73255; WO 00/47864; WO 01/173325 and WO 01/81801.
A horizontal spool tree assembly is provided for supporting the production tubing string within a well below a blowout preventer defining the BOP bore. The blowout preventer is connected to one or more fluid control lines extending from the BOP to the surface. A workover string may be used for fluid communication between the BOP bore and the surface. The tree assembly includes a spool body for positioning below the BOP and defining a spool body central bore for receiving a tubing hanger. The spool body has a lateral production passageway extending laterally from the central bore to a production valve, i.e., a horizontal tree. The tubing hanger is sealed to the spool body and adapted for supporting the production tubing string therefrom. The tubing hanger preferably includes a tubing hanger central bore in fluid communication with the interior of the tubing string and a lateral tubing hanger production passageway extending laterally from the tubing hanger central bore for fluid communication with the lateral bore in the spool body. A workover flow path is provided extending axially through the tubing hanger from the spool body central bore below the tubing hanger to the spool body central bore above the tubing hanger, thereby providing fluid communication with the workover string to an annulus below the tubing hanger and surrounding the production tubing string. A workover valve is positioned within the workover flow path for controlling fluid flow between the bore in the spool body above the tubing hanger and the annulus below the tubing hanger and surrounding the production tubing string during a workover operation.
It is a feature of the tree assembly that the spool body may also include an annulus port in a spool body extending laterally from an annulus surrounding the production tubing to an annulus valve. The spool body may also include a crossover port extending laterally through the spool body above the tubing hanger, a crossover flow line extending from the crossover port to the production valve, and a crossover valve for controlling fluid flow along the crossover flow line.
It is a further feature of the invention that the production valve may be positioned within the spool body. Yet another feature of the invention is that the workover valve positioned along the workover flow path in the tubing hanger may be a valve for holding pressure in either direction.
Yet another feature of the invention is that the tree assembly may include a safety valve positioned along the production tubing string below the tubing hanger. The tree assembly may also include a first closure member positioned within the central bore in the tubing hanger, and a second closure member positioned above the tubing hanger for isolating the BOP bore from the crossover port in the spool body. The second closure member may thus be positioned between the first closure member and the BOP bore for isolating the first closure member from the BOP bore.
A significant advantage of the present invention is that components which make up the assembly are highly reliable. By providing a workover flow path extending axially through the tubing hanger, the body of the spool tree acts as another barrier to fluid flow, which is lacking for embodiments wherein the spool body includes a laterally extending workover port. In the vicinity of the production passageway in the spool body, a first barrier for workover fluid is the body of the tubing hanger itself, while the second barrier is the spool body. As a further advantage of the invention, the tree assembly may be used with conventional equipment commonly related to wellhead assemblies, including safety valves and closure members.
A related advantage of the production assembly is that fewer valves may be required compared to prior art production assemblies, especially outboard of the tree body.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.